Various detectors and systems have been developed to detect and indicate the presence of particles of combustion, or of a fire, or of an increase in temperature. Such systems generally use two or more conductors between a control panel or control unit, which is coupled to the individual detectors. In general, the individual detectors determine when an undesired condition is present, by comparing some parameter (such as current flow or voltage level) with a predetermined reference value. When the detector finds the reference value has been exceeded, this means the undesired condition is present and the detector latches in the alarm condition. Generally the control unit does not know the precise location of the alarmed detector, and after three or more detectors have gone into alarm, cannot recognize how many detectors are in the alarmed condition.
Such prior art detectors are generally not capable of having their sensitivity checked from the control panel, and having their sensitivity individually adjusted directly at ech detector without taking the system out of operation when such adjustment is necessary.
A serious shortcoming of prior art systems is that detectors are frequently made to snap into a base connection across the conductor pair. If the detector is removed and replaced by a cardboard form or some other mechanical unit to simulate detector presence, continuity along the conductor pair is maintained and the control unit does not "know" that the detector is in fact missing from the area.
It is thus a primary consideration of the present invention to provide a bidirectional, interactive fire detection system in which only a single conductor pair is required. In such a system it is desired to have the control panel or controller selectively address the individual devices, and to have each device respond when addressed and commanded to inform the controller of the individual device status.
Another important consideration is the provision of such a system in which the controller issues command signals to the addressed device, which command signals represent desired functions or actions to be taken by the selectively addressed device, which then accomplishes the functions or actions. It is desired to have the comparison and latching functions in the control panel, with the addressed device only communicating signal level information to the control panel, which then determines if an alarm condition is present.
It is another consideration to have each individual device selectively and remotely calibrated, without affecting the operation of such device or the remainder of the system during such adjustment.